You can change heading by doing following changes
<asp:gridview id="gvreceptionist" runat="server" xmlns:asp="#unknown">
AutoGenerateColumns="False"
onselectedindexchanged="gvdisplayappointments_SelectedIndexChanged">
<columns>
<asp:templatefield headertext="Your Header name here">
<itemtemplate>
</itemtemplate></asp:templatefield></columns></asp:gridview>